An HEV is a vehicle that has a propulsion system that consists of at least one electric motor or electric machine in combination with at least one other power source. Typically, the other power source is a gasoline or diesel engine. There are various types of HEVs depending on how the electric motor(s) and other power source(s) are combined with one another in order to provide propulsion for the vehicle, including series, parallel and compound HEVs.
Various hybrid powertrain architectures are known for managing the input and output torques of various propulsion systems in HEVs, most commonly internal combustion engines and electric machines. Series hybrid architectures are generally characterized by an internal combustion engine driving an electric generator which in turn provides electrical power to an electric drivetrain and to an energy storage system, comprising a battery pack. The internal combustion engine in a series HEV is not directly mechanically coupled to the drivetrain. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine, and the electric drivetrain may recapture vehicle braking energy by also operating in a generator mode to recharge the battery pack.
Parallel HEV architectures are generally characterized by an internal combustion engine and an electric motor which both have a direct mechanical coupling to the drivetrain. The drivetrain conventionally includes a shifting transmission to provide the necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide for continuously variable speed ratios by combining features from both series and parallel HEV powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions (i.e. input split, output split and compound split configurations) thereby enabling high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and two-mode operation.
Essentially all transmissions have a spring damper located between the engine and transmission. Although it is commonly just called a “damper,” it is actually built with springs that can be designed to dissipate energy. The spring damper decouples the rotating inertia of the engine from the rotating inertia of the transmission, thereby providing some level of isolation from high frequency oscillations (e.g. engine firing pulses are attenuated as they pass through the damper into the transmission). When an engine is “keyed-off”, the engine is passively stopped as the rotational energy is dissipated due to frictional losses and the operation of the spring damper.
The design of the spring damper is constrained by the mechanical packaging of the springs themselves. The spring design must meet conflicting criteria. The springs must be stiff enough (large spring constant, K) to accept the maximum torque of the engine within their free travel length. However, long springs can encounter problems in buckling and are very difficult to package. And the natural frequency of the system must be significantly lower than the desired idle speed (e.g., by about 1/√{square root over (2)} for attenuation), but the high stiffness (K) tends to drive the natural frequency higher (e.g., by √{square root over (K/M)}) relationship). A key difference between certain EVT transmissions and other transmissions is that the engine is continuously coupled through a spring damper to the transmission and its large inertias. There is no decoupling starting clutch (as in a manual transmission) or torque converter (as in an automatic transmission). These large inertias produce significant vibrational energy as the engine and transmission pass through a resonant frequency associated with the engine speed (e.g. the resonance speed of an compression engine/spring damper/transmission system is approximately 400 rpm for a six cylinder engine which permits sufficient attenuation by idle speed (i.e., 600 rpm and higher)), thereby affecting operator and passenger feel and perceived vehicle performance. Certain EVTs have the possibility of actively applying an opposing torque to the engine using one or more electric motors.
Therfore, it is desirable to develop operating modes for EVT powertrain systems that may be used to provide an active engine stop and shorten the time required to transition the engine through the powertrain resonant speed.